1. Field of the Invention
The invention relates to a controller, more particularly to a controller for use in a resonant direct current/direct current converter.
2. Description of the Related Art
As shown in FIG. 1, a half bridge LLC resonant direct current converter 11 is conventionally controlled by a frequency modulation controller 12, which detects an output voltage of the half bridge LLC resonant direct current converter 11 and varies frequency of a switching control signal when the operating condition is varying, so as to maintain the output voltage at the required level. The half bridge LLC resonant direct current converter 11 includes two switches (not shown), and the switching control signal includes first and second driving signals (as shown in FIG. 2).
However, since the frequency of the switching control signal varies according to the output voltage, the converter 11 has a range of variation of the switching control signal, making it easy to deviate from an optimal operating point, and in turn resulting in a large design margin in order to overcome uncertainty caused by the variation. Furthermore, the variable frequency characteristic inherent to the half bridge LLC resonant direct current converter 11 also increases the complexity of control.